Hi Jason
In the olden days from WWI and right through to Vietnam and even after that. The tac signs were painted either directly onto the vehicles or onto the small 6x6 size squares for the cars and Land Rovers etc or onto the 8 1/2" wide x 9" tall signs for the trucks.
When they started printing adhesive stickers, in the hundreds or thousands for the 1 Div etc, the stickers were a little smaller than the 6X6 size of the plate. However when units had individual stickers made for their unit/regiment, sometimes they were made the same size as the Divisional sticker but at other times they were printed in the full 6"x6" size. This meant that the divisional sticker on a vehicle may be as small as 5"X5" with the unit sign being 6"X6" like yours.